The invention relates to the separation of gas mixtures. It is particularly concerned with the separation of gas mixtures by dephlegmation, otherwise known as reflux condensation. Dephlegmation or reflux condensation is a method in which a gaseous mixture which is being separated by rectification is simultaneously heat exchanged with a fluid stream that is raised in temperature by the heat exchange and thereby condenses fluid being rectified to create a reflux flow for the rectification.
Dephlegmation has been used to separate ethylene from mixtures of hydrocarbons as is described by H. C. Rowles et al in "Ethylene recovery by cryogenic dephlegmation" see "Gas separation technology", 1989 pp 609 to 616 (Elsevier Science publishers). Dephlegmation has also been proposed for use in air separation. U.S. Pat. No. 2 963 872 discloses a process of producing oxygen-enriched air using dephlegmation. In this process, a conventional distillation column with trays is employed to separate the air into a first fluid enriched in oxygen and a second fluid enriched in nitrogen. The liquid phase of the first fluid is taken from the bottom of the distillation column and is passed through a valve so as to reduce its pressure. The resulting reduced pressure liquid is then passed downwardly through a set of heat exchange tubes located in the distillation column. Thus, some of the fluid being rectified in the distillation column is condensed thereby creating a reflux flow, while the liquid is itself evaporated and passes out of the heat exchange tubes for collection or use as a product. It is known in conventional air separation processes employing a rectification column to sub-cool the oxygen-enriched liquid stream over a similar temperature range to that spanned by the rectification column. There are a number of other cryogenic separation processes which also employ heat exchangers that operate over similar temperature ranges to that spanned by the rectification column. The method and apparatus according to the invention enables such heat exchange to be performed in the dephlegmator.